1. Field of the Invention
The present invention relates to an active matrix organic electroluminescent (OEL) display device that has an organic thin-film transistor (OTFT), and more particularly, to an active matrix OEL display device, that has an array structure, including a p-type OTFT that has an aperture ratio of approximately 100%.
2. Description of the Related Art
FIG. 1 is a plan view of a sub-pixel unit in a conventional active matrix electroluminescent (EL) display device, and FIG. 2 is a cross-sectional view of the sub-pixel unit of the display device taken along line P1 through P7 of FIG. 1.
Referring to the drawings, in conventional silicon thin-film transistors (TFTs) 110 and 150 that have a semiconductor layer 180 formed of silicon, the semiconductor layer 180 includes a source region and a drain region which are heavily doped by impurities. In addition, it includes a channel region formed between the above two regions. In addition, the silicon TFTs 110 and 150 include gate electrodes 111 and 151 that are insulated from the semiconductor layer 180 and located to correspond to the channel region, source electrodes 112 and 152 and drain electrodes 113 and 153 that contact the source region and the drain region.
The problem with these conventional silicon TFTs 110 or 150 is that they are more expensive, fragile, and cannot use a plastic substrate since that they are fabricated at high temperature of 300° C. or higher, for example.
Flat panel display devices such as liquid crystal displays (LCD) or electroluminescent displays (ELD) use TFTs as switching devices and driving devices to control and operate pixels. In order to make flat panel display devices large, thin, and flexible, researchers are trying to use plastic substrates instead of the typical glass substrate. However, manufacturing display devices with plastic substrates is difficult because the fabrication temperature is below what is necessary for conventional silicon TFTs.
Since an OTFT does not have the above manufacturing problems, active research has been performed to develop OTFTs that have an organic semiconductor layer.
FIG. 3 is a schematic cross-sectional view of an OEL display device that has a conventional TFT.
Referring to FIG. 3, an OEL device 210 and an OTFT 220 are formed on a substrate 200. The OEL device 210 includes a transparent electrode 211, an organic light emitting layer 212, and a metal electrode 213 that are sequentially formed on the substrate 200. The OTFT 220 includes a gate electrode 221 formed on the substrate 200, a dielectric layer 222 formed on the gate electrode 221, an organic semiconductor layer 223 formed on the dielectric layer 222, and a source electrode 224 and a drain electrode 225 that are disposed on both sides of the organic semiconductor layer 223 on the dielectric layer 222. The drain electrode 225 is connected to the transparent electrode 211 and the organic light emitting layer 212 of the OEL device 210.
However, since the OEL device 210 is disposed adjacent to the OTFT 220, the OEL device 210 has low aperture ratio due to the size of the OTFT 220. When the aperture ratio is low, the light emitting intensity of each unit pixel of the display device must be increased, thus reducing the lifespan of the display device.
In order to solve the above problem, Korean Patent Publication No. 2003-0017748 discloses an active matrix OEL display device, in which an OTFT and an OEL device are stacked in a vertical direction. FIG. 4 is a cross-sectional view of an OEL display device that includes the OTFT disclosed in the above Patent Publication.
Referring to FIG. 4, an OEL device 310 and an OTFT 330 disposed on a substrate 300 are separated by a first insulating layer 320. The OEL device 310 includes a transparent electrode 311, an organic light emitting layer 312, and a metal electrode 313 sequentially formed on the substrate 300. The OTFT 330 includes a gate electrode 331 formed on the first insulating layer 320, a second insulating layer 332 formed on the gate electrode 331, a source electrode 334 and a drain electrode 335 formed on the second insulating layer 332, and an organic semiconductor layer 333 connected to the source and drain electrodes 334 and 335. In addition, the source electrode 334 is connected to the metal electrode 313.
However, the above example is merely an OEL device that includes one OTFT, not an array of a plurality of OEL devices and a plurality of OTFTs. In addition, complex processes are required to fabricate the OTFT 330 with such a complex inverted coplanar structure. Therefore, it is difficult to form the active matrix OEL display device that can be applied in actual situation.